Generally, handover denotes a function which switches a conversation channel of a base station which currently provides a service, i.e., a source base station (or source node) to a conversation channel of a neighbor base station of another radio region, i.e., a target base station (or target node) when a mobile terminal moves from a radio region of a base station where the mobile terminal receives the service to a radio region of the neighbor base station. Here, a base station is the generic term for various communication nodes such as a base station, a repeater, a relay, a femto cell base station, an access point, and the like.
Such handover may be classified into hard handover, soft handover, semisoft handover, fast handover, and the like according to a type, an object, and a use of a service to a mobile terminal and a system.
A fast handover targets a reduction of handover delay due to a movement of a mobile terminal and a provision of a fast switching among base stations for a real time traffic such as a voice, a video, and the like. In a fast handover, one base station (or access point) which provides a service is switched to other base station during a preset time, and handover processing time varies according to a related service and generally is about hundreds of milliseconds.
In a fast handover, a mobile terminal performs a Random Access Procedure (RAP) in order to access a target base station. This random access procedure is classified into a contention-based random access procedure and a contention-free random access procedure. The contention-based random access procedure denotes a procedure in which a plurality of mobile terminals select one of random access channels and competitively perform an access to a target base station, and the contention-free random access procedure denotes a procedure in which an arbitrary mobile terminal performs an access to a target base station using a previously allocated random access channel without a collision with other mobile terminals. Hereinafter, a handover procedure using a conventional contention-free random access procedure will be described with referent to FIG. 1.
FIG. 1 illustrates a conventional network-centric handover procedure using contention-free random access.
As illustrated in FIG. 1, the conventional network-centric handover procedure using contention-free random access will be followed.
Source base station 120 is a base station which provides a service to a mobile terminal 110, and transmits information of base stations which are located nearby the source base station 120 to the mobile terminal 110 which intends to perform handover. The information includes at least one base station unique information, cell ID, prefix information, and the like. The source base station 120 transmits a command to measure at least one of strength, an Signal to Noise Ratio (SNR), and an Signal to Interference plus Noise (SINR) of a signal received from base stations included in the information to the mobile terminal 110 (S111).
After receiving the measurement command from the source base station 120, the mobile terminal 110 measures at least one of strength, an SNR, and an SINR of a signal through a signal received from at least one base station which is nearby located (S112). The mobile terminal 110 transmits a measured result to the source base station 120 (S113). The source base station 120 decides whether or not to handover for the mobile terminal 110 and a target base station using the measured result received from the mobile terminal 110 (S114). The source base station 120 transmits a handover request message to a target base station 130 (S115). The handover request message is a message which the mobile terminal 110 transmits to a base station where the mobile terminal 110 intends to handover, i.e., the target base station 130. The handover request message includes information which a mobile terminal requires in order to perform a handover such as information on the mobile terminal, information of the source base station 120, and the like. After receiving a handover response message from the target base station 130 (S116), the source base station 120 transmits the received handover response message to the mobile terminal 110 (S117). The handover response message includes information on a channel for the mobile terminal 110 to access the target base station 130. The channel is contention-free random access channel.
The mobile terminal 110 performs a synchronization procedure with the target base station 130 where the mobile terminal 110 intends to handover (S118), and performs a contention-free random access procedure. The mobile terminal 110 reconfigures connection to a target base station through a Radio Resource Control (RRC) message (S120).